The development of plastic blood collection bags in the 1960's facilitated the separation of donated whole blood into its various components, thereby making platelet concentrates available as a transfusion product. The separation of a single unit of donated whole blood, about 450 ml in USA practice, into its components is accomplished by use of differential sedimentation. The unit of whole blood is collected in a plastic blood collection bag with integral satellite transfer bags, and is separated by centrifugation into red cell concentrate and platelet rich plasma. The platelet rich plasma is transferred to an empty attached satellite bag, and the sedimented red cells are sparated from the collection system. The platelet rich plasma is centrifuged at an increased G force which divides it into platelet concentrate (PC) and platelet poor plasma, which are separated by transfer to a satellite bag. The final PC should contain on average not less than 5.5.times.10.sup.10 platelets in 50 to 70 ml of plasma, or approximately 10.sup.6 platelets per microliter (.mu.l). The component separation must take place within 6 hours of whole blood collection. One unit of platelet concentrate usually increases the platelet count of a 70 kg individual by 5000-10,000/.mu.l. The usual dose administered to a thrombocytopenic adult ranges from 6-10 units of concentrate.
Platelets can also be prepared using a specialized blood component collection procedure called apheresis. Using a variety of continuous or discontinuous flow devices, platelets are collected from a single donor. With this method, whole blood is removed from the donor, and centrifuged by ex-vivo technique into its component parts. The desired component, platelets in this instance, is harvested and the remainder of the autologous blood is returned to the donor. This procedure allows collection of multiple units from one donor. Typically the 2 to 3 hour apheresis procedure will produce a platelet product containing 3.times.10.sup.11 platelets, equivalent to 6-8 units of random donor platelets. Donors providing single-donor platelet concentrates are usually HLA-matched to the recipient. Single-donor platelets are generally given to patients who are immunologically unresponsive to transfusions of random donor platelet concentrates, or to individuals who are considered to be candidates for bone marrow transplantation.
Platelets serve two major functions in maintaining hemostasis. First, they adhere to injured blood vessel walls, aggregate and form a hemostatic plug. Second, they participate in fibrin formation by releasing platelet factor 3 and promoting coagulation factor mediated hemostasis. Platelets also release vasoactive amines, cationic proteins, nucleotides and enzymes, as well as thromboxane A.sub.2 which induces vasoconstriction, and promotes platelet aggregation via its inhibitory effect on cyclic AMP formation.
Platelet transfusions are indicated for treatment of bleeding due to thrombocytopenia secondary to inadequate production of platelets by the bone marrow, a condition known as amegakaryocytic thrombocytopenia. This bone marrow hypoplasia may be due to chemotherapy, tumor invasion or primary aplasia. For example, a patient with acute leukemia may be thrombocytopenic at diagnosis or become thrombocytopenic secondary to chemotherapy or radiation procedures. Patients with adequate numbers of platelets but abnormal congenital platelet function may have a range of responses resulting in mild to severe hemorrhagic disease, for example Glanzmann's Thrombasthenia. Patients may also experience platelet functional disorders secondary to a plasma abnormality, such as von Willebrand's disease or uremia. Platelet transfusions may also be used in patients with thrombocytopenia associated with massive blood replacement secondary to trauma, or for patients undergoing surgical procedures necessitating large amounts of blood like open heart surgery. Patients who have ingested aspirin may also demonstrate a transient platelet dysfunction and require platelet transfusion support for emergency surgical procedures.
The use of platelet concentrates has and continues to rapidly increase. This is demonstrated by the last survey conducted by the American Blood Commission. This survey indicates that the national use of platelet concentrates increased from 0.41 million in 1971 to 2.86 million in 1980, a 6 fold increase. By contrast, the use of packed red cells during the same period went from 6.32 million to 9.99 million units, only a 1.5 fold increase. Random donor platelets were being prepared from less than 6 percent of the whole blood units collected in 1971 compared to almost 20 percent in 1980. The proportion of donated whole blood units used to prepare platelet concentrates in 1987 is approximately 70-80 percent. Future demands may exceed the available blood supply, and, indeed, present demand already does so in some localities.
There are several reasons for this accelerated platelet use, including more aggressive use of chemotherapy with resultant prolonged periods of bone marrow aplasia. The availability of platelet components and more aggressive use of platelet transfusion permit the use of these aggressive chemotherapy programs.
The transfusion of platelet concentrate is not without risk for those patients receiving both acute and chronic transfusion support. Chills, fever and allergic reactions may occur in patients receiving acute as well as chronic platelet therapy. Repeated platelet transfusions frequently leads to alloimmunization against HLA antigens, as well as platelet specific antigens. This in turn decreases responsiveness to platelet transfusion. Leucocytes contaminating platelet concentrates, including granulocytes and lymphocytes, are associated with both febrile reactions and alloimmunization leading to platelet transfusion refractoriness. Another life-threatening phenomenon affecting heavily immunosuppressed patients in Graft versus host disease. In this clinical syndrome, donor lymphocytes transfused with the platelet preparations can launch an immunological reaction against the host, i.e. the transfusion recipient, with pathological consequences. Another potential consequence of platelet transfusion is the transmission of bacterial, viral, and parasitic infectious diseases.
Growing evidence suggests that leucocyte depleted platelet concentrates decrease the incidence of febrile reactions and platelet refractoriness. Leucocyte depleted blood components should also be evaluated for a potential role in reducing the potential for Graft vs. host disease. Leucocyte depletion of platelet preparations may also diminish the transmission of certain viruses (e.g. AIDS and CMV).
Platelet preparation contain varying amounts of leucocytes. Platelet concentrates prepared by the differential centrifugation of blood components will have varying leucocyte contamination related to the time of centrifugation and G force used. Leucocyte contamination is also influenced by the choice of apheresis technique used to harvest the component. While the dose of contaminating leucocytes necessary to cause a febrile reaction or elicit platelet refractoriness in repeated transfusion remains unknown, Stec et.al (Stec, N., Kickler, T. S., Ness, P. M. and H. G. Braine, Effectiveness of Leukocyte (WBC) Depleted Platelets in Preventing Febrile Reactions in Multi-Transfused Oncology Patients, American Association of Blood Banks, San Francisco, (Abstract No. 598), Nov. 3-7, 1986) and Dan and Stewart (Dan, M. E., and Stewart, S., Prevention of Recurrent Febrile Transfusion Reactions Using Leukocyte Poor Platelet Concentrates Prepared by the "Leukotrap" Centrifugation Method, American Association of Blood Banks, San Francisco, (Abstract No. 597), Nov. 3-7, 1986) have demonstrated that leucocyte removal efficiencies of 81 to 85% are sufficient to reduce the incidence of febrile reactions to platelet transfusions. Several other recent studies report a reduction in alloimmunization and platelet refractoriness at levels of leucocyte contaminaton &lt;1.times.10.sup.7 per unit. The level of leucocyte contamination in conventional platelet preparations is generally at a level .gtoreq..times.10.sup.8. The existing studies, therefore, suggest at least a two log (99%) reduction of leucocyte contamination is required. More recent studies suggest that a three log (99.9%) or four log (99.99%) reduction would be significantly more beneficial. An additional desired criterion is to restrict platelet loss to about 15% or less of the native platelet concentrate.
Centrifugal methods are available which reduce the number of leucocytes contaminating platelet preparations. These methods have often proved unsatisfactory because they result in an unaccpetable platelet loss. Centrifugation in conjunction with the use of specially constructed pooling bags reduces the concentration of leucocytes by approximately a single log. The technique is expensive and labor intensive.
The use of laboratory filters to remove contaminating leucocytes from platelet preparations has in some instances yielded 2 log leucocyte removal efficiencies with platelet recoveries averaging 90%, however, in most studies employing laboratory filters, unacceptably high platelet losses have been reported. The experience with the use of laboratory filters to deplete platelet preparations of leucocytes has shown the procedures to be inconsistent in their performance. Further, the use of these devices is labor intensive and results in reduced shelf life for a unit of conventionally collected platelets. Because of the reduced shelf life, those units are not recommended for bedside use.